1. Field of the Invention
The present invention relates to a blow molding apparatus and a method for manufacturing large-volume hollow bodies of plastics material having multiple-layer walls.
2. Description of the Prior Art
Co-extrusion methods for manufacturing hollow bodies of the above-described type of plastics material having a maximum volumetric size of about 20 liters are known. The volumetric size of the hollow bodies to be manufactured is limited in this method by the length of the continuously extruded preform.
For the manufacture of articles having large volumes, preforms are required which have higher weights and, thus, greater lengths. However, in carrying out the continuous extrusion method, these preforms are extended during the extrusion in such a way that a defined wall thickness distribution in the article is no longer obtainable. In addition, due to the longer mold cooling times of hollow bodies having thick walls and large volumes, the preform cools off in the ambient air to such an extent that a problem-free welding of the mash seams can no longer be guaranteed. Thus, technological problems limit the use of the known method to hollow bodies having relatively small volumes.
Hollow bodies having large volumes and including barrier layers for improving the impermeability of the bodies are to be used as motor vehicle tanks, barrels, containers and storage vessels.
Accordingly, attempts have been made in the past to develop suitable manufacturing methods for producing hollow bodies having large volumes.
A particular difficulty is the uniform formation of several different layers of material in a material accumulation chamber as it is required for the discontinuous blowing process. It is not possible to simply transfer the previously known blow head constructions for single-layer hollow bodies. This is because, in the production of hollow bodies having multiple-layer walls, each of the individual material layers must be extruded in the blow head, must be joined together to form a multiple-layer melt composite and must be ejected in such a way that the uniform thickness of each individual layer is ensured.
In accordance with a known method, the individual melts are extruded outside of the blow head into separate accumulation chambers and the layers are joined in a subsequently arranged nozzle gap. The following difficulties are encountered in the use of this method.
First, it is extremely difficult to control the method in order to adjust the output speeds of the individual accumulation chambers in such a way that the necessary thicknesses of the layers can always be obtained in a repeatable manner uniformly over the entire length of the article to be manufactured and in accordance with the desired wall thickness profiles of the articles.
Second, joint lines occur in each material layer because each material melt must flow during the ejection process around the mandrel with increased shear velocity. Particularly in the case of plastics material of high molecular weight, this leads to weak points in the finished articles.
In addition, since the thicknesses of the individual layers are influenced by the output speed from the individual accumulation chambers as well as by the control for the wall thickness, the necessary adjustment of both of these parameters relative to each other poses a problem with respect to control and regulation. Accordingly, this known method is not satisfactory.
In accordance with another known method, the melts are conducted into accumulation chambers of individual tubular plungers. Each of these tubular plungers is driven by a hydraulic or mechanical drive. Also in this case, the adjustment of the individual output speeds relative to each other poses a problem with respect to control. Although joint lines are avoided when this method is used, a very expensive and complicated apparatus is required for manufacturing, for example, an article having a five-layer wall structure composed of inner carrier layer, coupling agent, barrier layer, coupling agent and outer carrier layer.
It is, therefore, the primary object of the present invention to provide a method and an apparatus of the above-described type which can be used for manufacturing articles having walls with any chosen number of layers, wherein the method is easy to control and the apparatus is not complicated and expensive.